1. Field of the Invention
The present invention relates generally to flow valves and particularly, but not by way of limitation, to a cut-off valve which may be installed on a fluid flow line while the line is under pressure.
2. Description of Related Art
In repairing or modifying the plumbing in a building, it is often necessary to shut off flow before proceeding with the work. Ideally, only the flow to the immediate work area would be shut off and all other plumbing would remain in service. However, there is often no shut-off valve at a point in the flow line which can be closed and cause minimal interruption of service. In such a case, flow must be shut off at the nearest upstream valve and service is interrupted at points other than the immediate work area.
U.S. Pat. No. 4,552,170 issued to Margrave discloses a valve which can be inserted in an existing line without disconnection of the line. The Margrave valve, which functions like a gate valve, is closed by rotating a threaded valve stem to position an expandable seal across the line.
One drawback to the Margrave valve is the wear on the expandable seal. As the valve is opened and closed, the expandable seal is exposed to wear against the cut portion of the line and the body of the valve.
Another problem with the Margrave valve is the difficulty in providing sufficient cutting force to cut through the line and in overcoming the fluid pressure of the line during the cutting process.